Hydraulic energy storage system



Jan. 5, 1965 .1. v. BouYoucos 3,163,985 1 HYDRAULIC ENERGY STORAGE SYSTEM Filed July 51, 1962 2 Sheets-Sheet l H/GH PRESSURE \l8 RESERVOIR V 7l BY ATTORNEY Jan 5, 1965 J. v. BouYoucos 3,163,985

HYDRAULIC ENERGY STORAGE SYSTEM Filed July 31, 1962 2 Sheets-Sheet 2 mk ,8 H/GH PRESSURE ff RESERVO/R /la REsERvo/R 15 LOAD x36'coNrRo1. VALVE 3,163,985 HYDRAUUC ENERGY SGRAGE SYSTEM Alohn V. Bouyoucos, liti Blossom 'Circie E.,

Rochester it), Nfi.' iiiied July 3l, it-e2, Ser. No. 213,739 17 laims. (Cl. oil- 51) This invention relates to a hydraulic energy storage vpower supply which uses the sea pressure head at moderate to great submergence depths as a reservoir of high pressure or potential energy. In combination with a further reservoir maintained or initially set at a pressure that is low relative to sea pressure head, large energy delivery rates may be achieved and sustained over substantial periods of time. The system of the invention enables significantly greater quantities of total energy to be put to work than is practical with conventional hydraulic energy storage systems. The'invention has particular applicability as a power supply for deep-planted hydroacoustic transducers as described in copending application for US. Letters Patent Serial No. 151,516, filed November 10,

' sacrificing pressure regulation. As a 'result,.,the practical 1961, and is equally well suited for deep submergence emergency propulsion systems and for o-ther undersea devices which demand high energy rates for reasonable time periods. v

Conventional hydraulic energy storage systems are known which are based on the potential energy of a gas under compression. Such systems may be used, for example, to supply peak power demands in hydraulic systems wherein the average power requirements are relatively low. These systems commonly use an accumulator which is simply a pressure vessel into which oil or some other hydraulic uid can be pumped against a compliant gas, usually separated from the hydraulic fluid by either a piston or ilexible barrier. During a typical operating cycle, the accumulator is charged by introducing hydraulic uid into the accumulator at low energy rate by means of a hydraulic pump. This operation gradual- 1y decreases the volume within the accumulator occupied by the gas and increases the gas pressure exerted on the fluid in the accumulator. A pressure difference is thus established between the high pressure at the accumulator and the low pressure reservoir of the system. Later in the cycle, the uid may be extracted from the high pressure ,accumulator at high energy rate (high flow rate) over a relatively short period of time; the fluid so removed passes through the load device, such as an acoustic energy generator, to accomplish useful work.

Since gas-loaded accumulators depend upon the compression of gas for stored energy, their performance obeys the well-known gas law. During expansion or compression, the gas law states that P Vn: constant where P is the pressure of the gas, V its volume, and iz is a coeicient whose value lies between 1 and 1.4 depending on whether the volume change occurs under predominantly isothermal or predominantly adiabatic conditions. that, for systems of the prior art, one cannot tolerate a significant volume change upon discharge without having the extraction of energy from the precharged accumulator be accompanied by a significant internal pressure change. This pressure change, besides being undesirable in itself for many types of hydraulic loads, will be associated with a decrease in available energy, inasmuch as the available or potential energy is proportional to pressure.

A novel feature of the system according to the invention is that, in contrast to conventional hydraulic energy storage power supplies wherein only a small fraction of the accumulator content may be used if reasonably constant pressure is desired across the load, nearly the entire volume capacity of the system may be utilized without From the above relation, it is at once evident storage capability for aA given size. power supply is increased by nearly an order of magnitude; over thatI achieved by the conventional accumulator arrangement.

In accordance with the invention,` the relatively constant pressure energy available by virtue of the head or" sea water in existence above the submerged power supply l is employed in conjunctiony with alow pressure reservoir whose internal pressure is maintained at aA low value in` in a typical.. arrangement, a iirst reservoir is connected inthe high comparison Vwith the ambient sea pressure.

pressure side of the system and includes a vessel having two regions separated by a ilexible member which performs as a barrier tok isolate hydraulic fluid from sea water. One of-these regions is adapted to receive hydraulic iiuid, while the other region is exposed to sea water at ambient pressure.` A second or low pressure reservoir is locatedv on the low pressure side of the system and typically takes the form of'a pressure vessel` which can support the ambient sea pressure, and which includes a further llexible barrier isolating a region containing gas such as nitrogen from a region containing hydraulic lluid. The latter .region is connected to the low pressure side of the hydraulic system. v

Located between the two reservoirs is normally a load which is driven by the pressure differential between the two reservoirs. Connected also between the two reser-` voirs is, typically, a pump which can extract hydraulic iiuid from the low pressure reservoir and deliver it at relatively low energy rate to the high pressure reservoir.

A normal operating cycle might proceed as follows. During a time interval when the load is not energized, meaning that no hydraulic ow through the load device occurs, the pump removes residual uid existing in the low pressure reservoir, and delivers this uid to the high pressure reservoir, leaving the available volume of the low pressure reservoir predominantly gas-filled. The load device may then be energized allowinghydraulic fluid to llow from the high pressure reservoir through the load to the low pressure reservoir. The rate of energy delivery to the load will depend upon the pressure differential between the two reservoirs and the hydraulic ow rate demanded by the load device.

When the low pressure reservoir is nearly lled with hydraulic fluid and the gas therein compressed to a nominally minimum volume, the load ow is stopped and the pump again begins to exhaust the fluid now contained in the low pressure reservoir, returning it to sea pressure head.

The load may accept energy from the reservoir system at high energy rate forY a relatively short interval ottime, and the pump may recharge the system' at a low energy rate over a correspondingly longer interval of time. The

ratio of the discharge to charge time is normally calledthe duty cycle of the system, and is inversely proportional to the ratio of energy delivery rate to energy charge rate.

In contrast with conventional accumulator energy storage techniques wherein the stored potential energy is dependent on gas pressure and will therefore change in proportion to the volume of iluid extracted from the accumulator, in the System of the present invention the stored potential energy resides in the constant sea pressure head which, `for all practical purposes, constitutes an iniinite reservoir of potential energy. The low pressure reservoir is intended, for purposes of the invention, to act as a pressure release. lts potential energy per unit volume, even under conditions of minimum nominal gas volume, is small, compared with the potential energy per unit volume associated with the sea pressure head. For purposes of illustration, it may be assumed that the storage system is located in the vicinity of 3090 feet below the surface of the sea where the ambient pressure is 1500 Patented 'p.s.i. and further that the,V pressure reservoir 'is at atmospheric pressure (15 psi.) when'the system is charged (i.e., lwhen the low pressure reservoir is primarily gas-filled). If upon energizingthe load, iiow intovthe low pressure reservoir continues untill the gas volume has been reduced to 10% of its initial volume, then the internal pressure of the gas will have increased fror'n 15 p.s.i. to approximately 150 Vp.s.i. The iinal pressure in the low reservoir is'thus only 10% of the total available pressure from the seay pressurejhead, yet 90% of the volume ofthe low pressurel reservoir has been displaced by working fluid. In conventional accumulavtor practice, only 10% of the gas 'volume within the accumulator maybe utilized if the load pressure is not to change more than 10% This example illustrates the significantly greater total energy capacity of a hydraulic energy storage system of a. given physical size which can utilize a source of constant potential or pressure energy suchas the sea pressure head,

and which incorporates .a highly compliant low pressure reservoir which, because of its low initial pressure, can be iilled with Huid to nearly its full volume without significantly altering the available pressure head.

in some instances it may be practical to employ sea water as the operative huid; in'these circumstances the barrier and associated vessel on the high pressure side of the system may be removed. y

Y Whereas the low pressure reservoir has been described as including a compliant gas to provide pressure release characteristics, in some applications where discharge time requirements are particularly short, the relatively smaller compliance of a liquid may be utilized. In this instance, the storage system becomes an all liquid system with the attendant advantages of greater potential reliability and longer life.

ln applications where long term repetitive cycling is unnecessary, the' pump circuitry may be eliminated and the storage supply may be submerged to design depth from the surface with the submerged low 'pressure reservoir at atmospheric pressure. The operation of submerging the source automatically establishes the potential energy head which is to be used and the equipment may be either expended or brought back to the surface for recharge at the end of the discharge cycle. Y

Other objects and features of the invention will become apparent upon reference to the following description and drawings wherein FIG. l is a schematic diagram of an energy storage power supply using the sea pressure head as a high pressure reservoiry FIG. 2 is a cross-sectional view of a modiiication of the system shown in FIG. 1; FIG. 3 is a diagram of a system using sea water as the operative tluid for the load; and

rFIG/4 isa diagram of a system wherein the pump circuitry shown in FGS. 1-3 is eliminated and the potential energy head is established by the operation of submerging the system. Y

Referring to the'drawing, the energy storage power supply of F16. 1 includes a low power pump Ill for cirn culating hydraulic huid, a load l5 through which hydraulic iiuid iiows and across which the system supply pressure is to be developed, and a pair of reservoirs 13 and 27 which are submerged at approximately'the same depth in the sea or some other large body of liquid. Each ol these reservoirs takes the form of a vessel into which a hydraulic duid can be introduced to displace either a compliant gas or sea water, the gas or sea water normally being separated from the hydraulic fluid by a sealed piston or by a closed ilexible diaphragm. The reservoir 18 is connected in the high pressure line 19, while the reservoir 27 is connected in the low pressure or discharge line 20. A portion of the wall of reservoir 18 is apertured to permit entrance of sea water at ambient pressure into compartment 21` This reservoir further includes a ilexible escasas internal pressure of the low barrier 23 made of a thin sheet of rubber, neoprene plastic or the like which isolates the hydraulic duid in compart-k ment 24 from the sea water, but which permits sea pressure head toV be existent at all times at the upstream or inlet port or terminal 25 of the hydraulic load 15.

" hydraulic fluid in fluid compartment 3l from the gas-filled compartment 3d. Flexible barrier 33 is also made of a compliant nonpervious material such as used in the flexible barrier Z3. v from the tluid compartment 31 of reservoir 27 until the internal pressure of reservoir 27 is near sea level (atmospheric) pressure, and delivers the hydraulic fluid at sea pressure head to reservoir 18. An unloading valve 38 and check valve 39 may be interposed in the hydraulic circuit of the pump to unload the pump once the desired pressure in reservoir 27 is achieved. Normally, the unloading valve 3E and check valve 39 will allow ow from the pump to be delivered directly from reservoir 27 to high pressure reservoir 18. However, when the internal pressure of reservoir 27 has been reduced to some prescribed value relative to sea pressure head, the unloading valve diverts the pump flow to return path 43 at negligible pressure drop, and check valve 39 blocks the hydraulic fluid in reservoir i8 at sea pressure head from owing back into the unloaded pump circuit. The unloading valve 38 senses vthe total pressure ditierence between the two reservoirs 1S and 27 by means of pilot connection 42 to the high pressure line 19 and return connection 43 to the low pressure line 58.

Load l5 may include its own control circuitry, operated by electrical connections 1100, for example, which could be used to shut ott load flow during the charge cycle. Alternatively, separate control elements such as solenoid valve 36 and/ or ow control valve 41 may be included in the discharge line Ztl to provide tlow control and tlow switching functions.

During a typical operating cycle, once the reservoir 27 has been essentially evacuated of hydraulic fluid by pump l0, the system is in a charged condition, the pressure in reservoir 27 is at a predetermined minimum value, and the pump is unloaded by means of unloading valve 39. Either by applying excitation to the load iiow control circuit or to the solenoid of valve 36, flow through the load circuit will occur at a pressure drop corresponding essentially to the pressure difference between reservoirs l land 27. Flow power will then be delivered to the load at a level dependent upon the aforesaid pressure drop and the controlled rate of tluid volume tlow. At some time prior to the exhaustion of hydraulic tluid from reservoir l or to the filling of reservoir 27 with hydraulic fluid, the load flow is shut ot.

Following the initiation of the discharge cycle, and as reservoir 27 begins to lill with hydraulic fluid, thereby causing its internal pressure to increase, unloading valve 33 transfers the pump output from the unloading line 43 to the charging circuit 59 feeding the high pressure reservoir 18. The charging cycle may thus commence prior to the completion oi the discharge cycle, and continue following the completion of the dischargeV cycle vuntil reservoir 27 is again essentially evacuated of hydraulic duid and the system is fully charged.

It the pump 1.6 extracts fluid from reservoir 27 essentially continuously at a low energy rate, while the load i5 accepts energy from the power supply periodically at high energy rate for short time intervals, the electrical power supplied to the pump need be only approximately the peak power delivered to the load times the fractional duty cycle.

Although the unloading valve 38 and check valve 39, as shown in FIG. l, or suitable equivalents thereof, would normally be employed in circumstances Where the system can be fully charged prior to the initiation of the next The pump l0 removes hydraulic fluidY discharge cycle, their inclusion in the hydraulic circuit is by no means essential for all operating modes. For eX- ample, if the discharge repetition rate is constant, it may be founddesirable to let the pump operate continuously and to choose the pumping rate, discharge ilow rate, repetition rate and duty cycle so that the discharge cycle occurs immediately following the charging of the system to a prescribed valve (i.e., following the removal of a predetermined amount of uid from the low pressure reservoir). Alternatively, a simple pressure differential switch sensing the pressure difference between the two reservoirs might be employed to initiate and terminate a discharge cycle. In this instance, the pump could also beturned on or ott as controlled by the pressuredilerential switch. Y l l Atypical structural arrangement for use in a system such as illustrated schematically in FIG. l is shown in FIG. 2f The elements of FIG. 2 corresponding to those in FIG. 1 are denoted by like reference numerals. A pair of generally hemispherical shells 6l and 62 are joined together to form a low pressure reservoir 27. Reservoir 27`includes a -lleXible barrier` for dividing the reservoir into an upper portion 34 containing a gas, such as nitrogen, and a lower portion 3l adapted to receive hydraulic iiuid through a port 5S. An annular cylindrical high pressure reservoir le, secured Yto the shell d2 o reservoir 27 as by bolts 66, contains two compartments 2l and 24 separated by a flexible barrier 23. The outer portion of the wall of reservoir l is apertured to admit sea water under pressure into compartment 2l. Compartment 24 is filled with hydraulic huid and is subjectedto the sea pressure head through the flexibility of barrier 23.

A pump l@ is driven by an appropriate motor 7tlcentrally mounted within the central cylindrical region "il to the inner wall '.72 of reservoir 18. When control valve 36 is closed, hydraulic huid is removed by pump 10 from the compartment 3l of low pressure reservoir 27 by way of port 57 and uid conduit 5S exposed to the central region 71. The tluid then ows through the pump lil, thence through unloading valve 33 and check valve 39 and passage 77 into the hydraulic fluid compartment 2d or high pressure reservoir 1.8. Subsequently, when control valve 36 is open, the hydraulic huidk passes out through line i9, load l5, and control valve 3d to the line 26 leading to the low pressure reservoir 27.

The hydraulic system of FIGS. l and 2 has been illustrated as a closed system with barriers isolating the operative hydraulic fluid from sea water and the entrained gas in the low pressure reservoir. This procedure may generally be preferable as the operative liuid may then be selected to provide optimum characteristics in terms of lubricity, viscosity and corrosion protection from the point ofview of the pump, control and load elements. In some instances, however, it may be practical to employ sea water as the operative tluid and in these circum stances the barrier 23 and high pressure reservoir 18 may be removed to admit sea water directly into the hydraulic system; a system of this type is shown in FlG. 3. In i the system of FIG. 3, 'the entire high pressure reservoir 11S is receptive of sea water md the lluid compartment 31- of low pressure reservoir 27 contains sea water rather than hydraulic fluid. For simplicity, the pump and lluid control means has been shown in FIG. 3 by a reference numeral 161. It should be understood that many variations o-f pumps and valving are feasible. The pump and uid control means lul exhaust the sea water from the low pressure reservoir 27 directly into the sea through apertures in the high pressure reservoir lli.y The exhaust pump` and liuid control means lill set or maintain the low pressure reservoir Z7 in a state of low pressure relative to a high hydrostaticV pressure on the perforated reservoir 113. The difference in huid pressure between the low pressure reservoir 2'7 and the high pressure reservoir U8 is a measure of available lluid energy for the load l5.

Whereas the low pressure reservoir 27 of FIGS. 1-3 has been described as including a compliant gas to provide pressure release characteristics, in some applications where discharge time requirements are particularly short, theV relatively smaller compliance of a `liquid may be utilized.y

In this instance, the storage sys-tem becomes an all liquid system with the attendant advantages of lgreater potential reliability and longer life. Such a storage system lwouldy differ from that shown in FIGS. 1 3 only in'thatthe barrier 33 in the low pressure reservoir 27 oL FlGS. Al1-3l and the gas in the region 3d of the low pressure reservoir would be eliminated so that the entire volume of reservoir 27 may be filled with the'operative lluid. y

The energy storage system of FIGS. l and 2 has been illustrated with typical charging circuitry, namely, apump and unloading valve arrangement, to enable repetitive charge and discharge cycles` to occur. The pump may be riven by a submersible Velectric motor, connected' by a cable to an electric power source which may be at a shore or ship location. Alternatively, submerged, unattended power sources, such as a nuclear reactor or internal combustion engine, may be employed. In the latter case, in the event mechanical power lis available directly from the power source, direct coupling to the pump may be appropriate.

In applications where long term repetitive cycling is not necessary, the pump circuitry may be eliminated. For` example, as shown in FIG. 4, the storage supply including the high pressure reservoir T18 could be submerged to design depth from the surface with the submerged low pressure reservoir 27 which vmay be at atmospheric (sea level) pressure. The opera-tion of submerging the'system automatically establishes the potential energy head which subsequently is to employed. The control valve 36 regulates the flow of iluid from the high pressure reservoir l through the load l5 to the low pressure reservoir Z7. The Viow o'u-id from the high pressure reservoir `i8 to the low pressure reservoir 27 is considered the discharge cycle of the system. At the conclusion of the full discharge cycle, when the low pressure reservoir.

27 is iluid-lled, the equipment may bef expended or brought back to the surface for recharge.

What is claimed is:

l. In a hydraulic energy storage system for supplying hydraulic flow energy to a load having high and low pressure terminalsv upon passage of a fluid through saidV 2. In a hydraulic energy storage system for supplying hydraulic flow energy to a load having high and low pressure terminals uponpassage of a iiuid through said load, a ilurid medium of relatively large volume, means for hydraulically coupling said high pressure terminalV to said fluid medium in a region ofhigh pressure established at a predetermined depth within said fluid medium of relatively large volume, and a low pressure reservoir coupled to said low pressure terminal for receiving said uid from said load, said low pressure reservoir being submerged in said lluid medium at subtantially said predetermined depth.

3. In a hydraulic energy storage system for supplying hydraulic flow energy to a load having high and low pressure terminals upon passage `of a fluid through said load, a fluid medium of relatively large volume, means for` hydraulically coupling said high pressure terminal to annesseA depth, and control means fer of iiuid through Vsaidiload into said low pressure reservoir.

, 4. In a hydraulic energy storage system for supplying hydraulic iiow energy to a load having high and low pressure terminals uponl passage of iiuid' through saidv load,` a fluid medium of relatively large volume, means for hydraulically coupling'said high pressure terminal to said iiuid medium -in a region of high pressure established vat a predetermined depth within said fluid medium of' relatively large volume, a low pressure reservoir connected to said W pressure terminal -for receiving said fluid from said load, and control means for selectively permittingtransfer of fluid through said load into said low pressure reservoir.

5. In a hydraulic energy storage system for supplying l hydraulic flow energy to a load having high and low pressure terminals upon passage of'fiuid through said load, a 4iiuid of relatively large volume, means for hydraulically coupling said high pressure terminal to said iiuid medium in a region of high pressure established at a predetermined depth within said iiuid medium of relatively large volume, a low pressure reservoir connected to said lovv pressure terminal and submerged vin said fluid medium at substantially said predetermined depth, and control means for selectively permitting transfer of fluid from said iiuid medium through said load into said low pressure reservoir.

6. In a hydraulic energy storage system for supplying hydraulic iiow energy during one portion of an operating cycle to a load having high and low pressure terminals, said high pressure terminal being hydraulically coupled to a relatively large body of fluid at a predetermined depth at which a desired high pressure is attained, a low pressure reservoir connected to said low pressure terminal, iiuid from said body of iiuid being transferred through said load to said low pressure reservoir during one portion of said operating cycle, and a iiuid pump for transferring fiuid from said low pressure reservoir to said body of uid during the remaining portion of said operating cycle.

7. In a hydraulic energy storage system, a load adapted to convert hydraulic ow energy into energy of another form during a portion of each operating cycle in response to liuid iiow therethrough, said load having high and low pressure terminals, said high pressure terminal being opened and positioned in a relatively large body of fluid at a predetermined depth at which a desired high pressure is attained, a low pressure reservoir hydraulically coupled to said low pressure terminal and submerged to substantially said predetermined depth, and means for transferring uid through said load into said low pressure reservoir during said portion of said operating cycle, said reservoir having a uid compartment capable of volumetric change as iiuid is transferred to said reservoir.

8. In a hydraulic energy storage system, a load adapted to convert hydraulic flow energy into energy of another form during a portion of each operating cycle, said load having high and low pressure terminals, a high pressure reservoir and a low pressure reservoir hydraulically coupled respectively to said high pressure terminal and said 10W pressure terminal, said reservoirs each including hydraulic iiuid compartments capable of volumetric change, and means for controlling transfer of hydraulic fluid from said high pressure reservoir to said low pressure reservoir during said portion of said operating cycle until said low pressure reservoir is substantially, filled with hydraulic iiuid, said high pressure reservoir being submerged in a uid medium of relatively large volume to a depth at which a relatively high pressure exists, at least a portion of said high pressure reservoir being exposed directly to said fluid medium to maintain the pressure of said hydraulic iiuid in said fluid compartment of said high pressure reservoir substantially constant, the variation in pressure of the hydraulic fluid in said low pressure reservoir during said transfer of hydraulic iiuid being small comfor selectively permitting transpared with the variation in pressure of said hydraulic fluid in the fluid compartment of said high pressure reservoir.

9. In a hydraulic energy storage system, a load adapted to convert hydraulic fiovv energy into energy of another forrn once during each operating cycle, said load having high and low pressure terminals, a high pressure reservoir and a low pressure reservoir hydraulically coupled respectively to said high pressure terminal and said low pressure terminal, and means for transferring hyraulic iiuid from one reservoir to another and back during each operating cycle, said reservoirs having hydraulic iuid compartments capable of volumetric change as hydraulic fluid is transferred between reservoirs, said high pressure reservoir being submerged in a fluid medium of relatively large volume to a depth at which a relatively vhigh pressure exists, at least a portion of said high pressure reservoir being exposed directly to said iiuid medium to maintain the pressure of said hydraulic fiuid in said liuid compartment of said high pressure reservoir substantially constant.

10. ln a hydraulic storage system as recited in claim 9,

said reservoirs being joined together to form an integral` housing.

l1. In a hydraulic energy storage system having a high pressure line, a 10W pressure line and a load adapted to convert hydraulic fiow energy into energy of another form connected across said lines, a hydraulic pump for circulating iiuid in said system, a first reservoir connected to said low pressure line and exposed to discharge iiuid pressure from said load, a second reservoir connected to said high pressure line and submerged in a uid medium to a depth at which a relatively high pressure exists compared with said discharge Huid pressure, each of said reservoirs having a first portion adapted to receive said hydraulic fluid and a second portion separated from said rst portion by a flexible fluid barrier, said second portion of said iirstreservoir being filled With a compliant gas, and said second portion of said second reservoir communicating with said uid medium.

12. In a hydraulic energy storage system having a high pressure line and a low pressure line and a load adapted to convert hydraulic flow energy into energy of another form connected across said lines, a hydraulic pump for circulating a iirst hydraulic fiuid in said system, a rst reservoir connected to said low pressure line, a second reservoir connected to said high pressure line and submerged in a iiuid second medium to a depth dependent upon the desired pressure difference between said high and low pressure lines, each of said reservoirs having a iirst portion adapted to receive said first hydraulic iiuid and a second portion separated from said first portion by a tiexible uid barrier, said second portion of said rst reservoir being filled with a compliant gas, said second portion of said second reservoir being open to said second Huid medium, control valving means for permitting said first hydraulic iiuidto be transferred from the first portion of said first reservoir to the first portion of said second reservoir in the absence of a control signal, said valving means including means responsive to the attainment of a predetermined pressure dierence between the pressure at said iirst and said second reservoirs to discontinue said first hydraulic fiuid transfer and to unload said pump, and means responsive to the presence of said control signal for permitting said first hydraulic fluid to be removed from said first portion of the second reservoir and to be circulated through said load to the first portion of said tirst reservoir.

13. A system as recited in claim 12 wherein said load includes an electro-hydroacoustic transducer placed in operable condition when receptive of said control signal.

14. in combination, a hydraulic circuit through which is circulated a hydraulic iiuid, a hydraulic load having high and low pressure terminals connected in said hydraulic circuit, iirst and second reservoirs connected respectively to said low and high pressure terminals of said with a compliant gas at relatively low pressure compared with said high pressure, and means for permitting said uid to pass from said second reservoir to said first reservoir through said load during a portion of an operating cycle in response to a control signal, said load being adapted to convert hydraulic ow energy into energy of another form during llow of iluid therethrough.

l5. In combination, a hydraulic circuit through which is circulated a hydraulic fluid, a hydraulic load having high and low pressure terminals connected in said hydraulic circuit, control means for permitting said hydraulic iluid to pass through said load during a portion of an operating cycle in response to a control signal, said load being adapted to convert hydraulic tluid flow energy into energy of another form during ilow of said hydraulic fluid therethrough, first and second reservoirs connected respectively to said low and high pressure terminals of said load, said secondl reservoir being submerged in a iiuid medium to ahdepth at Which said high pressure exists, said reservoirs each being divided into rst and second compartments by a yielding fluid barrier, said first compartment of said reservoirs being arranged to receive said hydraulic fluid, said second compartment of said second reservoir being exposed to said iluid medium whereby the pressure within said rst compartment of said second reservoir is maintained substantially constant at said high pressure, said second compartment of said irst reservoir being lled with a compliant gas at a relatively low pressure compared with said high pressure, and means for transferring said hydraulic liuid from said rst reservoir to said second reservoir during absence of flow of said l@ hydraulic fluid-through said load until said pressure at said first reservoir falls to said low pressure.

16. In combination, a hydraulic circuit through which is circulated a hydraulic fluid, a hydraulic load having high and low pressure terminals connected in said hydraulic circuit, control means for permitting said hydraulic fluid to pass through said load during a portion of an operating cycle in response to a control signal, saidlload being adapted to convert hydraulic' fluid flow energy into energy of another form during llow of said hydraulic fluid therethrough, first and second reservoirs connected respectively to said low and high pressure terminals of said load, said second reservoir being submerged in a fluid medium to a depth at which said high pressure exists, said reservoirs each being divided into first and second compartments by a yielding liuid barrier, said lirst compartv r ent of said reservoirs being arranged to receive said hydraulic uid, said second compartment of said second reservoir being exposed to said uid medium whereby the pressure Within said iirst compartment of said second reservoir is maintained substantially constant at said high pressure, said second compartment of said first reservoir being filled with a compliant gas at a relatively low pressure compared with said high pressure, and means for transferring said hydraulic fluid from said l'irst reservoir to said second reservoir during absence of lloW of said hydraulic fluid through said load until said pressure at said rst reservoir falls to said low pressure, said control means including means for discharging said hydraulic fluid from the first compartment of said second reservoir through said load to said first reservoir during the presence of said control signal.

17. In a hydraulic storage system as recited in claim l5, said yielding fluid barrier being a thin sheet of a compliant nonpervious material.

`References Cited in the tile of this patent UNITED STATES PATENTS 2,599,577 Phillips May 30, 1950 2,597,050 Audemar May 20, 1952 2,644,307 Blair July 7, 1953 2,742,758 Kelly Api'. 24, 1956 

1. IN A HYDRAULIC ENERGY STORAGE SYSTEM FOR SUPPLYING HYDRAULIC FLOW ENERGY TO A LOAD HAVING HIGH AND LOW PRESSURE TERMINALS UPON PASSAGE OF A FLUID THROUGH SAID LOAD, A FLUID MEDIUM OF RELATIVELY LARGE VOLUME, MEANS FOR HYDRAULICALLY COUPLING SAID HIGH PRESSURE TERMINAL TO SAID FLUID MEDIUM IN A REGION OF HIGH PRESSURE ESTABLISHED AT A PREDETERMINED DEPTH WITHIN SAID FLUID MEDIUM OF RELATIVELY LARGE VOLUME, AND A LOW PRESSURE RESERVOIR COUPLED TO SAID LOW PRESSURE TERMINAL FOR RECEIVING SAID FLUID FROM SAID LOAD. 